Communication services providers are subject to very strict bandwidth usage spectrum constraints, such as technically mandated specifications and regulations imposed by the Federal Communications Commission (FCC), which requires that sideband spillage, namely the amount of energy spillover outside a licensed band of interest, be sharply attenuated (e.g., on the order of 50 dB). While such limitations are adequate for traditional forms of modulation such as FM, they are difficult to achieve using more contemporary, digitally based modulation formats, such as M-ary modulation. Attenuating the sidebands sufficiently to meet industry or regulatory-based requirements using such modulation techniques requires very linear signal processing systems and components. Although linear components can be implemented at a reasonable cost at relatively low bandwidths (baseband) of telephone networks, linearizing such components, in particular power amplifiers, at RF frequencies becomes a very costly exercise.
A fundamental problem is the fact that RF power amplifiers are inherently non-linear devices, and generate unwanted intermodulation distortion products (IMDs), which manifest themselves as spurious signals in the amplified RF output signal, separate and distinct from the RF input signal. A further source of RF amplifier distortion is the presence of spectral regrowth or spreading of a compact spectrum into spectral regions that do not appear in the RF input signal. The distortion introduced by an RF amplifier causes the phase and amplitude of its amplified RF output signal to depart from the respective phase and amplitude of the RF input signal, and may be considered as an incidental (and undesired) amplifier-sourced modulation of the input signal.
One brute force approach to linearize an RF power amplifier is to build the amplifier as large, high power device, and then operate the amplifier at a low power level, that is only a small percentage of its rated output power, where the RF amplifier's transfer function is relatively linear. An obvious drawback to this approach is the overkill penalty--a high cost and large sized RF amplifier.
Other prior art techniques include post-amplification feed forward correction, and pre-amplification, pre-distortion correction. In accordance with the former approach, error (distortion) present in the RF amplifier's output signal is extracted, amplified to the proper level, and then reinjected (as a complement of the error signal back) into the output path of the amplifier, such that (ideally) the RF amplifier's distortion is effectively canceled. In the second approach, a predistortion signal is injected into the RF input signal path upstream of the RF amplifier. The predistortion signal has a characteristic that has been predetermined to be ideally equal and opposite to the distortion expected at the output of the high power RF amplifier, so that when subjected to the transfer function of the RF amplifier, it should effectively cancel its anticipated distortion behavior. The predistortion mechanism may be made adaptive by extracting the error signal component in the output of the RF amplifier and adjusting the predistortion signal in accordance with the such extracted error behavior of the RF amplifier during real time operation, so as to effectively continuously minimize distortion in the amplifier's output.